Hemocyanin (KLH) was injected into the hind foot pads of rabbits. Six days later cell suspensions were prepared from the popliteal lymph nodes. KLH (1 ng--100 microgram) was added to the lymph node cells to induce the synthesis of antibody, protein, DNA and RNA from radioactive precursors. Agents which increase cyclic AMP levels--cholera enterotoxin, dibutyryl cAMP and prostaglandins E1 and E2--enhanced antibody but not total protein synthesis induced by optimal (1 microgram) and supraoptimal (100 microgram) amounts of KLH. Antibody synthesis by uninduced or suboptimally induced cultures was inhibited when cAMP levels were increased during the first 24 hr. Addition of these agents to cell cultures during peak antibody synthesis (72-120 hr) inhibited antibody and protein synthesis. Neither exogenous cyclic GMP nor various derivatives of cGMP affected antibody synthesis. Fractionation of these lymphoid cells on nylon fiber columns yielded cell populations which could be induced by KLH to synthesize antibody, but were not subject to enhancement of this synthesis by cAMP. Sperm whale myoblobin (Mb) was injected into the hind foot pads of rabbits. The popliteal lymph nodes were removed at least 30 days later. Various concentrations of Mb or various synthetic peptides corresponding to known sequences in Mb were added to cell cultures prepared from these nodes. Sequences from region 1-6, 15-22, 56-62, 94-100, 113-119, and 146-151 induced the synthesis of antibody to Mb, as well as of IgG and protein upon addition to these cultures. All of these sequences, with the exception of 1-6, induced the production of macrophage inhibitory factor by these Mb-primed lymph node cells.